We propose to create variable, reproducible, and reversible degrees of mitral regurgitation in anesthetized, open-chest dogs, with and without cardiogenic shock. The cardiodynamics of this hemodynamic stress will be studied before and during the intervention of uni-directional Intra-Aortic Balloon Pumping. High-fidelity measurements will be made of phasic pressures and flow, including instantaneous flow across the mitral valve, in order to evaluate the dynamics of the lesion and the effects of IABP. In particular, we will quantify the stroke volume, the fillingvolume, the regurgitant volume, and the relative changes in these parameters during balloon pumping. We hope to demonstrate the experimental verification of hypothesis that IABP should help MR by increasing the ratio of forward volume to regurgitant volume. We hope, thereby, to lay the groundwork for the eventual use of IABP on patients with acute mitral regurgitation. Inasmuch as the dynamics of ventricular septal defects, and the experimental techniques for studying this lesion are similar to the dynamics and study of MR, we propose also to simultaneously study the effects of IABP on VSD.